6000-series aluminium extruded material superior in paint-baking hardenability and method for manufacturing the same

ABSTRACT

This invention relates to a 6000-series aluminum extruded material containing magnesium (0.3% to 0.7% by mass), silicon (0.7% to 1.5% by mass), copper (0.35% or less by mass), iron (0.35% or less by mass), titanium (0.005% to 0.1% by mass), manganese (0.05% to 0.30% by mass), chrome (0.10% or less by mass), and zirconium (0.10% or less by mass) (provided that at least one transition element selected from the group consisting of manganese, chromium, and zirconium is contained in a total amount representing 0.05% to 0.40% by mass), with the balance comprising aluminum with inevitable impurities, such aluminium extruded material having a predetermined yield strength of 180 MPa or more with an increase of 60 MPa as a result of a thermal history corresponding to paint baking. Such 6000-series aluminium extruded profile is superior in paint-baking hardenability and thus the yield strength thereof can be secured to a level applicable to structural members of automobiles and the like with the use of a thermal history corresponding to paint baking.

TECHNICAL FIELD

The present invention relates to a 6000-series aluminium (Al—Mg—Si alloy) extruded material superior in paint-baking hardenability, which can be improved in terms of yield strength when subjected to a thermal history corresponding to paint baking as well as when subjected to thermal refining. The present invention can be applied to members subjected to a thermal history corresponding to paint baking, such as structural members of vehicles (e.g., automobiles), including frame structural members such as a side sill, a side member, a cross member, and a door frame.

BACKGROUND ART

In recent years, aluminium alloys used for structural members of automobiles and the like have been gaining attention from the viewpoint of global environmental protection. However, prices per unit weight of aluminium are more expensive than those of steel. With the use of aluminium, component costs tend to become expensive while weight reduction can be achieved. Thus, when aluminium alloys are applied to structural members of automobiles and the like, it is necessary to reduce the price of aluminium extruded material to be used.

Structural members of automobiles and the like can be manufactured by the following manufacturing steps of: extrusion molding→stretch straightening→cutting (aluminium extrusion step); secondary processing involving bending (depending on types of structural members of automobiles and the like); and thermal refining→painting→paint baking. During such steps of manufacturing structural members of automobiles and the like, an aluminium extruded material is subjected to two thermal histories corresponding to thermal refining and paint baking. In addition, when an aluminium extruded material in the state after secondary processing is treated by thermal refining, the load efficiency deteriorates, resulting in expensive product prices. Thus, it is preferable to abolish thermal refining (if possible) or thermal refining upon secondary processing so as to use the step of increasing the yield strength of an aluminium extruded material with the use of a thermal history corresponding to paint baking.

In addition, it is preferable that structural members of automobiles and the like have yield strengths at low levels upon secondary processing and the yield strengths of such structural members be secured to such an extent that the members are applicable when used as frame structural members such as a side sill, a side member, a cross member, and a door frame.

Further, when a 6000-series aluminium alloy, which is an Al—Mg—Si alloy, contains Mg₂Si and excessive Mg (or excessive Si) in total amounts of not less than 0.6 wt % in stoichiometric composition, so-called “negative effects” arise. In such case, the yield strength of such alloy after natural aging increases compared with such strength immediately after extrusion, while the yield strength after aging treatment decreases compared with cases in which such alloy is not allowed to stand at room temperature. Preferably, the aluminium extruded profile having paint-baking hardenability does not experience yield strength increase even when allowed to stand at room temperature and efficiently exhibits performance after being subjected to a thermal history corresponding to paint baking.

For the application of aluminium alloy plates, various methods have been suggested for the purposes of improving paint-baking hardenability with the use of a thermal history corresponding paint baking. Examples of such methods that have been disclosed include a method wherein the alloy content is adjusted and Be or B is added (JP Patent Publication (Kokai) No. 6-2063 A (1994)) and a method wherein the cooling rate after solution treatment is controlled (JP Patent Publication (Kokai) No. 9-176806 A (1997)).

Further, for the application of an aluminium extruded profile, JP Patent Publication (Kokai) No. 2004-204321 A discloses a method wherein working strain is introduced by stretch straightening, secondary processing, or the like following extrusion such that aging is accelerated.

Furthermore, JP Patent Publication (Kokai) No. 2002-235158 A discloses a method for producing an aluminum alloy extruded profile that is superior in bending workability and has paint-baking hardenability: wherein an aluminum alloy ingot containing Mg (0.3% to 1.3% by mass), Si (0.2% to 1.2% by mass), and Sn (0.01% to 0.3% by mass) with the balance comprising Al with inevitable impurities is preheated at 400 to 550° C. and subjected to hot extrusion molding, followed by cooling to 50° C. or less at a cooling rate of 50° C./min or more; and wherein the alloy is subjected to stabilizing treatment in the temperature range of 50 to 140° C. within 24 hours (hereafter abbreviated as “hr”) after the extrusion molding in a manner such that the alloy is retained for 0.5 to 50 hr while having a yield strength of 120 N/mm or less

DISCLOSURE OF THE INVENTION

When an aluminium extruded profile having paint-baking hardenability is applied to structural members of automobiles and the like, it is preferable that such aluminium extruded profile have a yield strength of 180 MP or more in view of protection of vehicles upon crashing.

In addition, according to the above conventional techniques, a method wherein the alloy content of Be, B, or the like is adjusted (JP Patent Publication (Kokai) No. 6-2063 A (1994)) requires complicated control of the content. Also, a method wherein the cooling rate is controlled (JP Patent Publication (Kokai) No. 9-176806 A (1997)) comprises complicated steps and causes cost increases when applied to a thick extruded profile. These conventional techniques are applied to aluminium rolled plates having a plate thickness of approximately 1 mm. Thus, if such techniques are directly applied to aluminium extruded profiles, such aluminium extruded profiles might not sufficiently exhibit paint-baking hardenability. Further, in the case of a method wherein working strain is introduced after extrusion (JP Patent Publication (Kokai) No. 2004-204321 A), it is difficult to control steps of stretch straightening and it is also difficult to carry out secondary processing due to an increase of yield strength as a result of work hardening. Furthermore, when stretch straightening is not carried out, it is necessary to carry out secondary processing. When the above methods are applied to structural members of automobiles and the like, the application is limited so that the methods cannot be applied depending on parts of automobiles and the like. Furthermore, stabilizing treatment must be carried out within 24 hr following extrusion molding in the case of the method disclosed in JP Patent Publication (Kokai) No. 2002-235158 A.

In view of the problems of the aforementioned conventional techniques, it is a technical objective of the present invention to provide an aluminum extruded profile superior in paint-baking hardenability, the yield strength of which can be secured to a level applicable to structural members of automobiles and the like with the use of a thermal history corresponding to paint baking (approximately 150 to 200° C.×0.3 to 0.5 hr).

The present inventors have found that the above problems can be solved with the use of: an aluminium extruded material obtained by retaining a 6000-series aluminium alloy (Al—Mg—Si alloy) with a specific composition at a predetermined temperature for a predetermined period of time immediately after extrusion molding and allowing the alloy to be subjected to a thermal history corresponding to paint baking; or an aluminium extruded material obtained by setting a specific billet temperature and a specific cooling rate immediately after extrusion in the steps of manufacturing such extruded material with the use of the above 6000-series aluminium alloy. This has led to the completion of the present invention.

That is, first, the present invention relates to a 6000-series aluminum extruded material containing: magnesium (0.3% to 0.7% by mass) and silicon (0.7% to 1.5% by mass) for the ensuring of strength; copper (0.35% or less by mass) for the ensuring of elongation; iron (0.35% or less by mass) for the ensuring of yield strength; titanium (0.005% to 0.1% by mass) for microcrystallization; and manganese (0.05% to 0.30% by mass), chrome (0.10% or less by mass), and zirconium (0.10% or less by mass) for tissue stabilization upon extrusion (provided that at least one transition element selected from the group consisting of manganese, chromium, and zirconium is contained in a total amount representing 0.05% to 0.40% by mass); with the balance comprising aluminum with inevitable impurities. Such 6000-series aluminium extruded material has a predetermined yield strength of 180 MPa or more with an increase of 60 MPa as a result of a thermal history corresponding to paint baking. The 6000-series aluminium extruded material of the present invention is an aluminium extruded material superior in paint-baking hardenability, which can be improved in terms of yield strength when subjected to a thermal history corresponding to paint baking as well as when subjected to thermal refining.

The 6000-series aluminium extruded material of the present invention can be obtained by the steps of:

(1) retaining an aluminium extruded material at 90±50° C. for 1 to 24 hr immediately after extrusion molding; and

(2) setting the billet temperature at 500° C. or more and the cooling rate at not less than 70° C./min for 4 minutes immediately after extrusion during manufacturing of the extruded material.

Secondly, the present invention relates to a method for manufacturing a 6000-series aluminium extruded material, wherein an aluminum alloy ingot containing magnesium (0.3% to 0.7% by mass), silicon (0.7% to 1.5% by mass), copper (0.35% or less by mass), iron (0.35% or less by mass), titanium (0.005% to 0.1% by mass), manganese (0.05% to 0.30% by mass), chrome (0.10% or less by mass), and zirconium (0.10% or less by mass) (provided that at least one transition element selected from the group consisting of manganese, chromium, and zirconium is contained in a total amount representing 0.05% to 0.40% by mass), with the balance comprising aluminum with inevitable impurities, is subjected to extrusion molding.

As described above, the 6000-series aluminium extruded material of the present invention can be obtained by the steps of:

(1) retaining an aluminium extruded material at 90±50° C. for 1 to 24 hr immediately after extrusion molding; and

(2) setting the billet temperature to 500° C. or more and the cooling rate at not less than 70° C./min for 4 minutes immediately after extrusion during manufacturing of the extruded material.

According to the present invention, a 6000-series extruded profile superior in paint-baking hardenability that has sufficient yield strength as a result of a thermal history corresponding to paint baking and a method for manufacturing the same are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C show steps of manufacturing automobile members with the use of conventional aluminium extruded profiles and the aluminium extruded profile of the present invention. In the figures, door frames were used as examples for comparison and explanation. FIG. 1A shows steps of manufacturing a conventional separate-type door frame. FIG. 1B shows steps of manufacturing a conventional integrated door frame. FIG. 1C shows steps of manufacturing the integrated door frame of the present invention.

FIG. 2 shows a cross section of a test piece.

FIG. 3 shows a thermal history corresponding to paint baking.

BEST MODE FOR CARRYING OUT THE INVENTION

FIGS. 1A to 1C show steps of manufacturing automobile members with the use of conventional aluminium extruded profiles and the aluminium extruded profile of the present invention. In the figures, door frames were used as examples for comparison and explanation.

FIG. 1A shows steps of manufacturing a conventional separate-type door frame. A billet (BLT) consisting of an aluminium member is subjected to extrusion steps (extrusion molding→stretch straightening→cutting), bending such as stretch bending, secondary processing involving welding with another aluminium member, and thermal refining (e.g., T5 (200° C.×3 hr)) in that order. Thereafter, a painting step is carried out and then the door frame is manufactured by paint baking at approximately 170° C.×0.3 hr.

FIG. 1B shows steps of manufacturing a conventional integrated door frame. A billet (BLT) consisting of an aluminium member is subjected to extrusion steps (extrusion molding→stretch straightening→cutting), secondary processing involving bending such as stretch bending, and thermal refining (e.g., T5 (200° C.×3 hr)) in that order. Thereafter, a painting step is carried out and then the door frame is manufactured by paint baking at approximately 170° C.×0.3 hr.

During the above steps of manufacturing conventional door frames, an aluminium extruded material is subjected to two different thermal histories corresponding to thermal refining and paint baking. In addition, when an aluminium extruded material in the state after secondary processing is subjected to thermal refining, the load efficiency deteriorates, resulting in high product costs.

Meanwhile, FIG. 1C shows steps of manufacturing the integrated door frame of the present invention. A billet (BLT) consisting of an aluminium member is subjected to extrusion steps (extrusion molding→stretch straightening→cutting), and secondary processing involving bending such as stretch bending. Thereafter, a painting step is carried out without thermal refining and then the door frame is manufactured by paint baking at approximately 170° C.×0.3 hr. As described above, according to the present invention, thermal refining is abolished such that the yield strength of an aluminium extruded material is increased with the use of a single thermal history corresponding to paint baking alone.

Unlike thermal refining, in the case of paint baking, a thermal history of approximately 170° C.×0.3 hr is used. Thus, the aging temperature of paint baking is lower than that of general thermal refining (e.g., 200° C.×3 hr). In addition, the retention time for paint baking is shorter than that for general thermal refining. In order to secure the applicable structural member yield strength, the density of an Mg₂Si precipitate that is deposited upon aging treatment is preferably equivalent to that obtained upon thermal refining, even in a case in which an aluminium extruded profile having paint-baking hardenability is treated at a low aging temperature and retained for a short period of time. The yield strength of a 6000-series aluminium alloy can be improved with the presence of such Mg₂Si precipitate. Therefore, although magnesium and silicon are contained in the case of the present invention, the upper limits of the magnesium and silicon contents are provided. This is because excessive magnesium and silicon contents can significantly inhibit extrusion moldability. Further, the presence of copper results in the improvement of yield strength and elongation. However, excessive copper content inhibits extrusion moldability and corrosion resistance. Furthermore, regarding iron, iron crystal is obtained upon casting and a bulky iron precipitate is deposited upon high-temperature heating, resulting in the decreased density of an Mg₂Si precipitate that is deposited upon aging treatment. Accordingly, inhibition of the increase in yield strength occurs upon aging treatment.

According to the present invention, an aluminium extruded profile having paint-baking hardenability and a method for manufacturing the same are established. Such aluminium extruded profile satisfies the scope of alloy contents allowing paint-baking hardenability to be efficiently exhibited without the inhibition of extrusion moldability of 6000-series aluminium alloy, and the yield strength of such aluminium extruded profile can be secured at 180 MPa or more with an increase of 60 MPa as a result of a thermal history corresponding to paint baking in view of protection of vehicles upon crushing.

Alloy contents of the aluminium extruded profile having paint-baking hardenability and the method for manufacturing of the aluminium extruded profile of the present invention will be described below.

[Magnesium and Silicon]

After extrusion, cooling is carried out such that magnesium and silicon form a supersaturated solid solution with aluminium. During subsequent aging treatment, an Mg₂Si precipitate is formed, resulting in the improvement of alloy strength. In order to secure the necessary yield strength of an aluminium extruded profile having paint-baking hardenability, the magnesium content is preferably 0.3% or more. However, excessive magnesium content causes a significant increase in deformation resistance upon extrusion molding. Thus, the magnesium content is preferably 0.7% or less. Therefore, the magnesium content is 0.3% to 0.7% and more preferably 0.4% to 0.6%.

Silicon is less likely to inhibit extrusion productivity even when the amount thereof is larger than that of magnesium. In addition, in order to secure the necessary yield strength of an aluminium extruded profile having paint-baking hardenability, the silicon content is preferably 0.7% or more. However, when the silicon content exceeds 1.5%, silicon is less likely to form a solid solution with aluminium as a result of cooling following extrusion. In addition, even with greater silicon content, extrusion productivity tends to be inhibited as with the case of magnesium. In view of the above, the silicon content is preferably 1.5% or less. Thus, the silicon content is 0.7% to 1.5% and more preferably 0.8% to 1.3%. [Copper]

Preferably, copper is contained for the ensuring of strength and elongation. However, excessive copper content causes a decrease in corrosion resistance. In addition, deformation resistance upon extrusion increases so that productivity tends to be inhibited. In view of the above, the copper content is 0.35% or less.

Upon casting, an intermetallic compound comprising iron is crystallized in a large amount, resulting in a decrease in alloy strength. Such intermetallic compound is bulky and contains silicon that constitutes an Mg₂Si precipitate which causes the improvement of yield strength upon subsequent aging treatment. Thus, the density of the precipitate decreases. In addition, excessive iron content causes a decrease in corrosion resistance. In view of the above, the iron content is 0.35% or less.

[Manganese, Chrome, and Zirconium]

Manganese, chrome, and zirconium have effects of inhibiting recrystallization upon extrusion and stabilizing the fibrous structure. However, chrome and zirconium significantly inhibit quenching sensitivity. With the presence of chrome and zirconium, a supersaturated solid solution is less likely to be formed during fan air cooling following extrusion in the case of an aluminium extruded profile that constitutes a structural member of automobiles and the like. During subsequent aging treatment, the density of an Mg₂Si precipitate that causes the improvement of yield strength decreases. In addition, zirconium is formed into an intermetallic compound with titanium upon casting, resulting in fewer effects of titanium microcrystallization and the occurrence of cracking upon casting.

Manganese is relatively less likely to inhibit quenching sensitivity and is likely to suppress recrystallization. In order to obtain recrystallization suppression effects, the manganese content must be 0.05% or more. However, when the manganese content is 0.30% or more, quenching sensitivity is inhibited, as with the cases of chrome and zirconium. In addition, a supersaturated solid solution is less likely to be formed during fan air cooling following extrusion in the case of an aluminium extruded profile that constitutes a structural member of automobiles and the like. During a subsequent aging treatment, the density of an Mg₂Si precipitate that causes the improvement of yield strength decreases.

In view of the above, the manganese content is 0.05% to 0.30%, the chrome content is 0.10% or less, and the zirconium content is 0.10% or less. The total content of at least one transition element selected from the group consisting of manganese, chrome, and zirconium is 0.05% to 0.40%. [Titanium]

With the presence of titanium, fine crystals can be obtained upon casting. However, the addition of titanium at an excessive amount results in saturation of effects that are obtained by addition of titanium. In view of the above, the titanium content can be 0.005% to 0.10%, more preferably 0.005% to 0.05%, and further preferably 0.005% to 0.03%.

[Inevitable Impurities]

Incorporation of inevitable impurities, such as a base metal used for casting of an aluminium alloy and an intermediate alloy comprising added elements, occurs via different routes. Different elements are incorporated; however, they hardly influence alloy properties as long as the content of a single element is 0.05% or less and the total content of such elements is 0.15% or less. In view of the above, the content of a single inevitable impurity is 0.05% or less and the total content of inevitable impurities is 0.15% or less.

[Manufacturing Method: (1) Retention at 90±50° C. for 1 to 24 hr Immediately After Extrusion Molding]

An aluminium extruded material having paint-baking hardenability refers to an aluminium extruded profile subjected to the manufacturing steps of: extrusion molding→retention at 90±50° C.×1 to 24 hr; painting; and a thermal history corresponding to paint baking. As a result of retention at 90±50° C.×1 to 24 hr following extrusion molding, nuclei (i.e., GPzones) of Mg₂Si precipitates are formed, and such precipitates are formed as a result of a subsequent thermal history corresponding to paint baking. Many GPzones can be formed as a result of retention at low temperatures. However, in the case of retention at less than 50° C., formation of GPzones requires a retention time of 24 hr or more, resulting in the deterioration of production efficiency. Thus, it is preferable that an aluminium extruded material having paint-baking hardenability be retained at 50° C. or more. In addition, in the case of retention at a temperature of more than 120° C., Mg₂Si precipitates excessively grow, resulting in an increase in yield strength. Thus, upon subsequent secondary processing, workability tends to be inhibited. Thus, the temperature is preferably retained at 120° C. or less. In view of the above, retention is carried out at 90±50° C.×1 to 24 hr and preferably at 70±10° C.×1 to 12 hr following extrusion molding. Such retention step at 90±50° C.×1 to 24 hr may be carried out in an atmosphere furnace, a water bath, or an oil bath following extrusion molding and air cooling. Alternatively, such aluminium extruded material may be allowed to stand to cool at a controlled temperature following extrusion molding so as to be retained under thermal insulation.

Further, natural aging (causing an increase in strength due to GPzone formation, which gradually takes place when an extruded profile is allowed to stand at room temperature following extrusion molding) occurs in conventional cases in which a step of retention at 90±50° C.×1 to 24 hr is not carried out. However, GPzones are formed during retention at 90±50° C.×1 to 24 hr. Thus, such retention step is also effective for inhibition of subsequent natural aging.

[Manufacturing Method: (2) Setting of the Billet Temperature at 500° C. or More and Setting the Cooling Rate at Not Less than 70° C./min for 4 Minutes Immediately After Extrusion During Manufacturing of an Extruded Material]

In the case of an aluminium extruded material having paint-baking hardenability, the billet temperature is set at 500° C. or more and the cooling rate is set at not less than 70° C./min for 4 minutes immediately after extrusion during manufacturing. Further, such aluminium extruded material having paint-baking hardenability refers to an aluminium extruded profile retained at 90±50° C.×1 to 24 hr immediately after extrusion molding and subjected to painting and a thermal history corresponding to paint baking. In general extrusion steps, the billet temperature must be retained at 600° C. or less so that the upper limit of the temperature is not predetermined. The billet heating temperature is set at 500° C. or more and the cooling rate is set at not less than 70° C./min for 4 minutes immediately after extrusion. Accordingly, a supersaturated solid solution, which is necessary for formation of nuclei (i.e., GPzones) of Mg₂—Si precipitates that are formed as a result of subsequent retention at 90±50° C.×1 to 24 hr, can be obtained. When the billet temperature is less than 500° C., a vacancy that is necessary for GPzone formation cannot be formed inside an aluminium matrix. In addition, when the cooling rate is less than 70° C./min, a vacancy disappears during cooling, or solute atoms in a solid solution are deposited in the form of precipitates. In such case, GPzones cannot be formed by subsequent retention at 90±50° C.×1 to 24 hr. Thus, the billet heating temperature is set at 500° C. or more and the cooling rate is set at not less than 70° C./min for 4 minutes immediately after extrusion. Accordingly, nuclei (i.e., GPzones) of Mg₂Si precipitates that are formed after retention at 90±50° C.×1 to 24 hr can be formed. Further, a greater number of GPzones can be formed by retention at low temperatures. However, in the case of retention at less than 50° C., GPzone formation requires a retention time of 24 hr or more, resulting in the deterioration of production efficiency. Thus, an aluminium extruded material having paint-baking hardenability is preferably retained at 50° C. or more. In addition, in the case of retention at temperatures above 120° C., Mg₂Si precipitates grow excessively, resulting in an increase in yield strength. Thus, upon subsequent secondary processing, workability tends to be inhibited. Thus, the temperature is preferably retained at 120° C. or less. In view of the above, retention is carried out at 90±50° C.×1 to 24 hr and preferably at 70±10° C.×1 to 12 hr following extrusion molding. Such retention step at 90±50° C.×1 to 24 hr may be carried out in an atmosphere furnace, a water bath, or an oil bath following extrusion molding and air cooling. Alternatively, such aluminium extruded material may be allowed to stand to cool at a controlled temperature following extrusion molding so as to be retained under thermal insulation.

The aluminium extruded profile having paint-baking hardenability of the present invention is preferably used as a solid-core or hollow profile. It may be in a rectangular tube form, a cylindrical form, or an irregular form.

EXAMPLES

Hereafter, Examples and Comparative examples of the present invention will be described.

[Manufacturing Method: (1) Retention at 90±50° C. for 1 to 24 hr Immediately After Extrusion Molding]

First, starting material contents were adjusted so as to achieve compositions of 6000-series aluminium alloys shown as test samples Nos. 1 and 2 listed in table 1. The starting materials were dissolved and melted into cylindrical ingots (diameter: 204 mm×length: 700 mm) having a size appropriate for extrusion. In addition, alloy contents listed in table 1 are expressed in analytical values. The value “0.00%” is shown for the effective digit. Subsequently, ingots were subjected to homogenization treatment at 560° C.×4 hr.

Next, extrusion molding of ingots (billets) subjected to homogenization treatment was carried out using an extrusion molding die at predetermined extrusion temperatures (billet heating temperatures) under cooling conditions listed in table 2. Thus, aluminium extruded profiles each having a cross section of a frame structural member shown in FIG. 2 were formed.

TABLE 1 Alloy content (wt %) No. Si Fe Cu Mn Mg Cr Zn Ti Test sample 1 1.10 0.18 0.20 0.08 0.59 0.00 0.00 0.01 Test sample 2 0.90 0.18 0.20 0.08 0.40 0.00 0.00 0.01

TABLE 2 Time for allowing Step of retention at low a sample to stand temperature following Retention Time for allowing a sample to stand Tensile extrusion Temperature time following low temperature treatment strain B.H. No. (hr) (° C.) (hr) (hr) (%) treatment 1 Example 1 12 70 12 6 2 B.H. 2 24 6 3 72 6 Comparative 4 168 6 example Example 5 12 12 6 12 24 7 12 72 8 12 168 2 Example 1 12 70 12 6 2 B.H. 2 24 6 3 72 6 Comparative 4 168 6 example Example 5 12 12 6 12 24 7 12 72 8 12 168

The obtained aluminium extruded profiles having paint-baking hardenability were evaluated by tensile experiments regarding yield strength, strength, and breaking elongation. Tensile properties were determined by collecting flat test pieces from the extruded profiles having paint-baking hardenability and examining the test pieces using a tensile tester (complying with the JIS standards) according to JIS-Z 2241. Regarding criteria, a yield strength of 180 MPa or more was designated with “◯” in view of protection of vehicles upon crushing. A yield strength of 180 to 150 MPa was designated with “Δ,” because application is possible depending on sectional design. In addition, a yield strength of less than 150 MPa was designated with “x.” Further, considering the cases involving secondary processing, differences in terms of yield strength before and after a thermal history corresponding paint baking of 60 MPa or more and of less than 60 MPa were designated with “◯” and “x,” respectively. Then, comprehensive judgment was carried out. Table 3 shows the evaluation results.

TABLE 3 Mechanical properties Mechanical properties Yield before B.H. after B.H. strength 0.2% 0.2% increase Tensile yield Tensile yield as a strength strength Elongation strength strength Elongation result Comprehensive No. (MPa) (MPa) (%) (MPa) (MPa) (%) Judgment of B.H. Judgment judgment 1 Example 1 258 119 22 300 211 17 ◯ 92 ◯ ◯ 2 277 138 22 297 204 17 ◯ 66 ◯ ◯ 3 283 145 21 296 206 16 ◯ 61 ◯ ◯ Comparative 4 284 147 21 296 204 17 ◯ 57 X X example Example 5 258 120 22 300 214 17 ◯ 94 ◯ ◯ 6 259 118 22 301 210 17 ◯ 92 ◯ ◯ 7 260 121 21 302 209 17 ◯ 88 ◯ ◯ 8 257 119 23 299 212 17 ◯ 93 ◯ ◯ 2 Example 1 211 90 24 262 182 18 ◯ 92 ◯ ◯ 2 232 110 24 253 176 18 Δ 66 ◯ Δ 3 235 113 23 251 173 18 Δ 60 ◯ Δ Comparative 4 248 135 24 249 170 19 Δ 35 X X example Example 5 212 95 24 262 184 17 ◯ 89 ◯ ◯ 6 215 96 23 264 183 18 ◯ 87 ◯ ◯ 7 213 94 24 261 181 17 ◯ 87 ◯ ◯ 8 212 93 24 263 185 18 ◯ 92 ◯ ◯

[Evaluation]

Test sample No. 1 is an aluminium extruded profile containing Si (1.10%), Cu (0.20%), Mg (0.59%), and Mn (0.08%). Test samples Nos. 1-1 to 1-3 corresponding to the Examples and a test sample No. 1-4 corresponding to the Comparative example were allowed to stand at room temperature for 12 to 168 hr following extrusion molding and retained at 70° C.×12 hr. Then, the samples were compared in terms of yield strength before and after bake hardening (hereafter abbreviated as “B. H.”) treatment. In addition, test samples Nos. 1-5 to 1-8 corresponding to the Examples were allowed to stand for 12 hr following extrusion molding, treated at 70° C.×12 hr, and further allowed to stand at room temperature for 12 to 168 hr. Then, the samples were compared in terms of yield strength before and after B. H. treatment.

As a result, in the cases of test samples Nos. 1-1 to 1-4, the yield strength after B. H. treatment became lower in inverse proportion to the length of time during which the relevant sample was allowed to stand at room temperature following extrusion molding. Yield strengths after B. H. treatment were 211 MPa, 204 MPa, 206 MPa, and 204 MPa, respectively. The results were judged as corresponding to “◯.” However, yield strength increases as a result of B. H were 92 MPa, 66 MPa, 61 MPa, and 57 MPa, respectively. The yield strength became lower in inverse proportion to the length of time during which the relevant sample was allowed to stand at room temperature. In the case of test sample No. 1-4 corresponding to the Comparative example, which had been allowed to stand at room temperature for 168 hr, the result was judged as corresponding to “x.” In the cases of the other samples, yield strength increases were 60 MPa or more as a result of B. H., and thus the results were judged as corresponding to “◯.” Accordingly, upon comprehensive judgment of test samples Nos. 1-1 to 1-4 corresponding to the Examples, in the cases of the samples that had been allowed to stand at room temperature for less than 168 hr following extrusion molding (test samples Nos. 1-1, 1-2, and 1-3), the results were judged as corresponding to “◯.” Meanwhile, in the case of the sample that had been allowed to stand at room temperature for 169 hr or more following extrusion molding (test sample No. 1-4), the result was judged as corresponding to “x.”

Further, test samples Nos. 1-5 to 1-8 corresponding to the Examples were allowed to stand for 12 hr following extrusion molding, retained at 70° C.×12 hr, and further allowed to stand at room temperature for 12 to 168 hr, followed by B. H. treatment. Yield strengths after B. H. treatment were 214 MPa, 210 MPa, 209 MPa, and 212 MPa, respectively. Thus, the samples were not affected as a result of being allowed to stand at room temperature after retention at 70° C.×12 hr. Therefore, the results were all judged as corresponding to “◯.” In addition, yield strength increases as a result of B. H. were 94 MPa, 92 MPa, 88 MPa, and 93 MPa, respectively. Thus, the samples were not affected as a result of being allowed to stand at room temperature after retention at 70° C.×12 hr. Therefore, the results were all judged as corresponding to “◯.” Accordingly, in the cases of test samples Nos. 1-5 to 1-8 corresponding to the Examples, the results were all judged as corresponding to “◯” upon comprehensive judgment.

Test sample No. 2 is an aluminium extruded profile containing Si (0.90%), Cu (0.20%), Mg (0.40%), and Mn (0.08%). Test samples Nos. 2-1 to 2-3 corresponding to the Examples and test sample No. 2-4 corresponding to the Comparative example were allowed to stand at room temperature for 12 to 168 hr following extrusion molding and retained at 70° C.×12 hr. Then, the samples were compared in terms of yield strength before and after B. H. treatment. In addition, test samples Nos. 2-5 to 2-8 corresponding to the Examples were allowed to stand for 12 hr following extrusion molding, treated at 70° C.×12 hr, and further allowed to stand at room temperature for 12 to 168 hr. Then, the samples were compared in terms of yield strength before and after B. H. treatment.

As a result, in the cases of test samples Nos. 2-1 to 2-4, the yield strength after B. H. treatment became lower in inverse proportion to the length of time during which the relevant sample was allowed to stand at room temperature following extrusion molding. Yield strengths after B. H. treatment were 182 MPa, 176 MPa, 176 MPa, and 170 MPa, respectively. In the case of the test sample No. 2-1, the result was judged as corresponding to “◯,” and in the cases of test samples Nos. 2-2 to 2-4, the results were judged as corresponding to “Δ.” Further, yield strength increases as a result of B. H. were 92 MPa, 66 MPa, 60 MPa, and 35 MPa, respectively. The yield strength became lower in inverse proportion to the length of time during which the relevant sample was allowed to stand at room temperature. In the case of test sample No. 2-4 corresponding to the Comparative example, which had been allowed to stand at room temperature for 168 hr or more, the result was judged as corresponding to “x.” In the cases of the other samples (test samples Nos. 2-1 and 2-3), yield strength increases as a result of B. H. were 60 MPa or more, and thus the results were judged as corresponding to “◯.”

Thus, upon comprehensive judgment, in the case of test sample No. 2-1 corresponding to the Example, which had been allowed to stand at room temperature for 12 hr following extrusion molding, the result was judged as corresponding to “◯.” Also, in the cases of test samples No. 2-2 and 2-3 that had been allowed to stand at room temperature for 24 to 72 hr following extrusion molding, the results were judged as corresponding to “Δ” because application is possible depending on sectional design although low yield strengths were exhibited after B. H. treatment. Meanwhile, in the case of the sample that had been allowed to stand at room temperature for 168 hr or more following extrusion molding (test sample No. 2-4), the result was judged as corresponding to “x.”

Further, test samples No. 2-5 to 2-8 corresponding to the Examples were allowed to stand for 12 hr following extrusion molding, retained at 70° C.×12 hr, and further allowed to stand at room temperature for 12 to 168 hr, followed by B. H. treatment. Yield strengths after B. H. treatment were 184 MPa, 183 MPa, 181 MPa, and 185 MPa, respectively. Thus, the samples were not affected as a result of being allowed to stand at room temperature after retention at 70° C.×12 hr. Therefore, the results were all judged as corresponding to “◯.” In addition, yield strength increases as a result of B. H. were 89 MPa, 87 MPa, 87 MPa, and 92 MPa, respectively. Thus, the samples were not affected by as a result of being allowed to stand at room temperature after retention at 70° C.×12 hr. Therefore, the results were all judged as corresponding to “◯.” Accordingly, in the cases of test samples No. 2-5 to 2-8 corresponding to the Examples, the results were all judged as corresponding to “◯” upon comprehensive judgment.

[Manufacturing Method: (2) Setting of the Billet Temperature at 500° C. or More and Setting the Cooling Rate at not Less than 70° C./min for 4 Minutes Immediately After Extrusion During Manufacturing of an Extruded Material]

First, starting material contents were adjusted so as to achieve the compositions of 6000-series aluminium alloys shown in table 4 (test samples Nos. 1 to 4). The starting materials were dissolved and melted into cylindrical ingots (diameter: 204 mm×length: 700 mm) having a size appropriate for extrusion. In addition, alloy contents listed in table 4 are expressed in analytical values. The value “0.00%” is shown for the effective digit. Subsequently, the ingots were subjected to a homogenization treatment at 560° C.×4 hr.

TABLE 4 Alloy content (wt %) No. — Si Fe Cu Mn Mg Cr Zn Ti Test sample 1 Example 1.10 0.18 0.20 0.08 0.59 0.00 0.00 0.01 Test sample 2 Example 0.90 0.18 0.20 0.08 0.40 0.00 0.00 0.01 Test sample 3 Comparative example 0.69 0.18 0.20 0.20 0.60 0.02 0.03 0.01 Test sample 4 Comparative example 0.44 0.18 0.00 0.00 0.49 0.00 0.00 0.01

Next, extrusion molding of ingots (billets) subjected to a homogenization treatment was carried out using an extrusion molding die at predetermined billet heating temperatures under cooling conditions listed in table 5. Thus, aluminium extruded profiles each having a cross section of a frame structural member shown in FIG. 2 were formed. Herein, under general cooling fan conditions listed in table 5, a 45-cm fan was set to rotate at 1680 rpm.

TABLE 5 Extrusion step Profile temperature Step of retention at low (Actual value) temperature Billet Cooling Cooling Immediately After fan Retention Tensile temperature fan rate after extrusion cooling Temperature time strain B.H. Thermal No. (° C.) setting (° C./min) (° C.) (° C.) (° C.) (hr) (%) treatment refining 1 Comparative 1 460 Medium 85.1 488.8 148.4 70 12 2 B.H. — example speed 2 480 ↑ 87.8 507.5 156.3 Example 3 500 ↑ 91.2 532.4 167.4 4 520 ↑ 90.7 553.2 190.3 Comparative 5 500 Low 44.6 529.0 350.4 example speed 2 Comparative 1 460 Medium 85.1 490.4 149.9 70 12 2 B.H. — example speed 2 480 ↑ 87.1 506.9 158.4 Example 3 500 ↑ 90.6 532.3 169.7 4 520 ↑ 90.3 552.8 191.4 Comparative 5 500 Low 43.5 528.5 354.4 example speed 3 Comparative 1 500 Medium 90.9 531.7 167.9 — — 2 B.H. — example speed 2 500 ↑ 89.9 531.5 171.8 — — — — T5 4 Comparative 1 500 Normal 89.8 530.0 170.6 — — 2 B.H. — example 2 500 ↑ 89.1 527.7 171.2 — — — — T5

Thereafter, the aluminium extruded material was retained at 70° C.×12 hr, allowed to stand at room temperature for 1 week, and subjected to a thermal history (B. H. treatment) corresponding to paint baking shown in FIG. 2. In this case, heat treatment corresponding to general thermal refining was not carried out.

The obtained aluminium extruded profiles having paint-baking hardenability were evaluated by tensile experiments regarding yield strength, strength, and breaking elongation. Tensile properties were determined by collecting flat test pieces from the extruded profiles having paint-baking hardenability and examining the test pieces using a tensile tester (complying with the JIS standards) according to JIS-Z 2241. Regarding criteria, a yield strength of 180 MPa or more was designated with “◯” in view of protection of vehicles upon crushing. A yield strength of 180 to 150 MPa was designated with “Δ” because application is possible depending on sectional design. In addition, a yield strength of less than 150 MPa with designated with “x.” Further, considering the cases involving secondary processing, differences in terms of yield strength before and after a thermal history corresponding paint baking of 60 MPa or more and of less than 60 MPa were designated with “◯” and “x,” respectively. Then, comprehensive judgment was carried out. Table 6 shows the evaluation results.

TABLE 6 Mechanical properties Mechanical properties Yield before B.H. after B.H. strength 0.2% 0.2% increase Tensile yield Tensile yield as a strength strength Elongation strength strength Elongation result Comprehensive No. (MPa) (MPa) (%) (MPa) (MPa) (%) Judgment of B.H. Judgment judgment 1 Comparative 1 232 109 22 233 152 17 Δ 43 X X example 2 250 114 22 261 171 17 Δ 57 X X Example 3 261 121 22 300 213 17 ◯ 92 ◯ ◯ 4 255 121 22 299 209 18 ◯ 87 ◯ ◯ Comparative 5 244 127 22 260 177 17 Δ 50 X X example 2 Comparative 1 192 81 24 203 133 18 X 52 X X example 2 206 84 25 213 147 18 X 63 ◯ X Example 3 211 93 24 261 178 18 Δ 85 ◯ Δ 4 210 90 25 262 184 18 ◯ 94 ◯ ◯ Comparative 5 201 95 24 225 155 17 Δ 60 ◯ Δ example 3 Comparative 1 197 95 20 210 105 16 X 10 X X example 2 229 197 9 — — — — 4 Comparative 1 161 71 20 164 85 14 X 14 X X example 2 264 233 11 — — — —

Test sample No. 1 is an aluminium extruded profile containing Si (1.10%), Cu (0.20%), Mg (0.59%), and Mn (0.08%). Test samples Nos. 1-1 to 1-4 were heated at different billet temperatures of 460° C., 480° C., 500° C., and 520° C., respectively, upon extrusion. The samples were compared in terms of yield strength before and after B. H. treatment. In addition, a sample heated at a billet temperature of 500° C. upon extrusion and treated at a cooling rate of less than 70° C./min for 4 minutes immediately after extrusion (test sample No. 1-5) was compared with the above samples. As a result, yield strengths after B. H. treatment were 152 MPa, 171 MPa, 213 MPa, 209 MPa, and 177 MPa, respectively. The samples treated at billet temperatures of less than 500° C. (test samples Nos. 1-1 and 1-2) and the sample treated at a cooling rate of less than 70° C./min for 4 minutes immediately after extrusion (test sample No. 1-5) had low yield strengths after B. H. treatment, and thus the results were judged as corresponding to “Δ.” In the cases of the other samples, yield strengths after B. H. treatment were 180 MPa or more, and thus the results were judged as corresponding to “◯.” In addition, yield strength increases as a result of B. H. were 43 MPa, 57 MPa, 92 MPa, 87 MPa, and 50 MPa, respectively. In the cases of the samples treated at billet temperatures of less than 500° C. (test samples No. 1-1 and 1-2) and the sample treated at a cooling rate of less than 70° C./min for 4 minutes immediately after extrusion (test sample No. 1-5), the yield strength slightly increased as a result of B. H. Thus, the results were judged as corresponding to “x.” In the cases of the other samples, yield strength increases as a result of B. H. were 60 MPa or more, and thus the results were judged as corresponding to “◯.” Accordingly, upon comprehensive judgment of test sample No. 1, in the cases of the test samples (Nos. 1-3 and 1-4) corresponding to the Examples, which had been heated at billet temperatures of 500° C. or more, the results were judged as corresponding to “◯.” Also, in the cases of the other samples, the results were judged as corresponding to “x.”

Test sample No. 2 is an aluminium extruded profile containing Si (0.90%), Cu (0.20%), Mg (0.40%), and Mn (0.09%). Test samples No. 2-1 to 2-4 were heated at different billet temperatures of 460° C., 480° C., 500° C., and 520° C., respectively, upon extrusion. The samples were compared in terms of yield strength before and after B. H. treatment. Further, a sample heated at billet temperature of 500° C. upon extrusion and treated at a cooling rate of less than 70° C./min for 4 minutes immediately after extrusion (test sample No. 2-5) was compared with the above samples. As a result, the yield strengths after B. H. treatment were 133 MPa, 147 MPa, 178 MPa, 184 MPa, and 155 MPa, respectively. The samples treated at billet temperatures of less than 500° C. (test samples No. 2-1 and 2-2) had low yield strengths after B. H. treatment, and thus the results were judged as corresponding to “x.” The sample treated at a billet temperature of 500° C. (test sample 2-3) had a low but acceptable yield strength value of 150 MPa or more, and thus the result was judged as corresponding to “Δ.” The sample treated at a billet temperature of 520° C. (test sample No. 2-4) had a sufficiently high yield strength after B. H. treatment, and thus the result was judged as corresponding to “◯.” In addition, yield strength increases as a result of B. H. were 52 MPa, 63 MPa, 85 MPa, 94 MPa, and 60 MPa, respectively. In the case of the sample treated at a billet temperature of 460° C. or less (test sample No. 2-1), the yield strength increase as a result of B. H. was less than 60 MPa, and thus the result was judged as corresponding to “x.” In the cases of the other samples (test samples Nos. 2-2, 2-3, 2-4, and 2-5), yield strength increases as a result of B. H. were 60 MPa or more, and thus the results were judged as corresponding to “◯.” Accordingly, upon comprehensive judgment of test sample No. 2, in the cases of the samples (Nos. 2-1 and 2-2) corresponding to the Examples, which had been heated at billet temperatures of 480° C. or less, the results were judged as corresponding to “x.” In the cases of the samples that had been heated at a billet temperature of 500° C. (test samples No. 2-3 and 2-5), the results were judged as corresponding to “Δ” because application is possible depending on sectional design although low yield strengths were exhibited after B. H. treatment. Also, in the case of the sample that had been heated at a billet temperature of 520° C. (test sample No. 2-4), the result was judged as corresponding to “◯.”

Test sample No. 3 corresponding to the Comparative example 1 is an aluminium extruded profile containing Si (0.59%), Cu (0.20%), Mn (0.20%), Mg (0.60%), and Cr (0.02%). The Si content does not fall within the scope of the present invention. The material was subjected to extrusion at a billet temperature of 500° C. and a cooling rate of not less than 70° C./min for 4 minutes immediately after extrusion, followed by B. H. treatment without treatment at 70° C.×2 h. The yield strength was 105 MPa and the yield strength increase as a result of B. H. was 10 MPa, and thus the results were judged as corresponding to “x.” When such material was subjected to general thermal refining, the yield strength was 197 MPa. Thus, the material can be applied depending on type of structural member used in automobiles and the like. However, such material might have poor paint-baking hardenability, resulting in cost increase.

Test sample No. 4 corresponding to the Comparative example is an aluminium extruded profile containing Si (0.44%) and Mg (0.49%). The Si content does not fall within the scope of the present invention. The material was subjected to extrusion at a billet temperature of 500° C. and a cooling rate of not less than 70° C./min for 4 minutes immediately after extrusion, followed by B. H. treatment without treatment at 70° C.×2 h. The yield strength was 85 MPa and the yield strength increase as a result of B. H. was 14 MPa, and thus the results were judged as corresponding to “x.” When such material was subjected to general thermal refining, the yield strength was 233 MPa. Thus, the material can be applied depending on type of structural member used in automobiles and the like. However, such material might have poor paint-baking hardenability, resulting in cost increase.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide a 6000-series aluminium extruded profile superior in paint-baking hardenability, the yield strength of which can be secured to a level applicable to structural members of automobiles and the like with the use of a thermal history corresponding to paint baking. The aluminium extruded profile of the present invention can be applied to members that are subjected to a thermal history corresponding to paint baking, such as structural members of vehicles (e.g., automobiles), including frame structural members such as a side sill, a side member, a cross member, and a door frame. 

1. A 6000-series aluminum extruded material containing magnesium (0.3% to 0.7% by mass), silicon (0.7% to 1.5% by mass), copper (0.35% or less by mass), iron (0.35% or less by mass), titanium (0.005% to 0.1% by mass), manganese (0.05% to 0.30% by mass), chrome (0.10% or less by mass), and zirconium (0.10% or less by mass) (provided that at least one transition element selected from the group consisting of manganese, chromium, and zirconium is contained in a total amount representing 0.05% to 0.40% by mass), with the balance comprising aluminum with inevitable impurities, such aluminium extruded material having a predetermined yield strength of 180 MPa or more with an increase of 60 MPa as a result of a thermal history corresponding to paint baking.
 2. The 6000-series aluminum extruded material according to claim 1, which has been retained at 90±50° C. for 1 to 24 hours within 72 hours after extrusion molding.
 3. The 6000-series aluminum extruded material according to claim 1, which is obtained by setting the billet temperature at 500° C. or more and the cooling rate at not less than 70° C./min for 4 minutes immediately after extrusion during manufacturing of the extruded material.
 4. A method for manufacturing a 6000-series aluminium extruded material having a predetermined yield strength of 180 MPa or more with an increase of 60 MPa as a result of a thermal history corresponding to paint baking, comprising carrying out extrusion molding of an aluminium alloy ingot containing magnesium (0.3% to 0.7% by mass), silicon (0.7% to 1.5% by mass), copper (0.35% or less by mass), iron (0.35% or less by mass), titanium (0.005% to 0.1% by mass), manganese (0.05% to 0.30% by mass), chrome (0.10% or less by mass), and zirconium (0.10% or less by mass) (provided that at least one transition element selected from the group consisting of manganese, chromium, and zirconium is contained in a total amount representing 0.05% to 0.40% by mass), with the balance comprising aluminum with inevitable impurities.
 5. The method for manufacturing a 6000-series aluminium extruded material according to claim 4, wherein the extruded material is retained at 90±50° C. for to 24 hours immediately after the extrusion molding.
 6. The method for manufacturing a 6000-series aluminium extruded material according to claim 4 or 5, comprising setting the billet temperature at 500° C. or more and the cooling rate at not less than 70° C./min for 4 minutes immediately after extrusion during manufacturing of the extruded material. 